The invention relates to an active filter device provided for compensating harmonic distortion in a supply line, said device comprising a signal processing unit having a first input for receiving an electric current and a voltage, applied on said supply line and being provided for determining, on the basis of said received current and voltage, a load current value for each harmonic frequency of a predetermined first set of harmonic frequency components of a load current on said supply line, said signal processing unit being further provided for generating for each of said harmonics, a control signal on the basis of said load current value, said device further comprising a power module, having a second input for receiving said control signal, and a power source for generating for each of said harmonics a compensation current.
Such an active filter device is known from the article "A new high performance active harmonic conditioner based on the current injection mode" of Serge Bernard and Gerard Trochain published in Power Quality Bremen--November 1995 proceedings p. 243-252. The known active filter device is connected in parallel to the supply line, which connects a power source to a load. The load generates harmonic currents, which are injected into the supply line and which will consequently disturb the power supplied by the source. The active filter device uses power electronics to generate harmonic components, in order to cancel the harmonic currents on the supply line. The signal processing device monitors the electric current and voltage, applied on the supply line, and determines for each harmonic frequency of the first set of harmonic frequencies, the load current on the supply line. On the basis of this load current a control signal is generated, which is supplied to the power module. The latter being provided for generating, under control of the control signal, the compensation current for each harmonic frequency of said first set. The compensation current is then injected into the supply line, in order to compensate the load current. In such a manner, the current on the supply line is "cleaned" from the harmonic currents, injected by the load.
A drawback of the known active filter device is, that the reaction of the supply line upon application of the compensation current, is not accurately determined because the topology of the supply line changes all the time.